Dresden 2026 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 53: Nonequilibrium Quantum Systems II (joint session TT/DY)

TT 53.7: Talk

Wednesday, March 11, 2026, 16:45–17:00, HSZ/0105

Consistent quantum treatments of nonconvex kinetic energies — Christina Koliofoti, Mohammad Atif Javed, and •Roman-Pascal Riwar — Peter Grünberg Institut (PGI-2), Forschungszentrum Jülich, 52428 Jülich, Germany

The task of finding a consistent relationship between a quantum Hamiltonian and a classical Lagrangian is of utmost importance for basic, but ubiquitous techniques like canonical quantization and path integrals. Nonconvex kinetic energies (which appear, e.g., in nonlinear capacitors or classical time crystals) pose a fundamental problem: the Legendre transformation is ill-defined, and the more general Legendre-Fenchel transformation removes nonconvexity essentially by definition. Arguing that such anomalous theories follow from suitable low-energy approximations of well-defined, harmonic theories, we show that seemingly inconsistent Hamiltonian and Lagrangian descriptions can both be valid, depending on the coupling strength to a dissipative environment. There occurs a dissipative phase transition from a nonconvex Hamiltonian to a convex Lagrangian regime, involving exceptional points in imaginary time. Our approach thus resolves apparent inconsistencies and provides computationally effcient methods to treat anomalous, nonconvex kinetic energies.